1. Field of the Invention
The present invention relates to Light Emitting Diode (LED) lighting, and more particularly to circuitry for driving LED lighting and LED lighting containing such circuitry.
2. Description of the Related Art
Electrical lighting is widely used all around the world, and has been for close to a century. For most of this time, the dominant form of electrical lighting has been incandescent light bulbs. However other forms of light bulbs are becoming more popular in recent years.
Incandescent light bulbs pass an electrical current through a wire (called the filament) which causes the wire to heat up until it becomes white hot and emits light. Incandescent light bulbs are cheap, reliable, and very widely used, however most of the energy that they consume is emitted as waste heat rather than as useful light. Mainly because of this waste heat, the majority of the energy consumed by an incandescent light bulb is wasted, and the efficiency of incandescent light bulbs is typically as low as 2%-3%.
Compact fluorescent light bulbs (also known as CFL bulbs) use a fluorescent tube which is folded to fit in the space of an incandescent light bulb, allowing the use of a CFL light bulb in place of an incandescent light bulb. The fluorescent tube generates light through gas discharge by using electricity to excite a mercury vapor, which then emits ultraviolet light, which in turn is converted to visible light by a phosphor that coats the inside of the tube. CFL light bulbs are significantly more efficient than incandescent light bulbs, with typical efficiencies of 10% or so.
Unfortunately, CFL light bulbs are more complicated to manufacture than incandescent light bulbs, as they require a ballast to limit the current in the fluorescent tube after startup. Also, dimmer switches are commonly used with light bulbs, and CFL light bulbs are not very compatible with dimmer switches. Complications such as these make the manufacture of CFL light bulbs more expensive than incandescent bulbs—typically a CFL light bulb will cost several times more than an incandescent light bulb with the same level of light output. Finally, CFL light bulbs contain small amounts of mercury which is hazardous to humans and which makes it more difficult to dispose of them safely.
Light Emitting Diode light bulbs (also known as LED light bulbs) are even better than CFL light bulbs. The LEDs used in LED light bulbs are rapidly increasing in efficiency and light output, which allows LED light bulbs to get better over time. At typical efficiencies of 15% or so, LED light bulbs already offer higher efficiencies than CFL bulbs, and this efficiency advantage is expected to increase over time. Also, LED light bulbs do not contain hazardous materials, and are more flexible than CFL light bulbs. For example, LED light bulbs typically work better with dimmer switches which are commonly used by consumers.
Unfortunately, LED light bulbs are currently more expensive than CFL bulbs, and they are significantly more expensive than incandescent bulbs. Even with today's prices, the energy savings over a multi-year period of use is sufficient to justify the extra cost of LED light bulbs. This is especially true given that LED light bulbs typically have a longer lifetime than CFL bulbs, and a significantly longer lifetime than incandescent bulbs. The prices of LED bulbs are expected to decline in the future. As these prices decline, any remaining consumer resistance to using LED bulbs will reduce, and LED light bulbs are expected to become the dominant type of light bulb.
LED light bulbs use light emitting diodes (commonly referred to as LEDs) to generate light. LEDs operate by running electrical current through a forward biased diode junction which then emits monochromatic photons of light. These monochromatic photons are then converted to broad spectrum white light by use of a phosphor. The LEDs typically need to be supplied with a voltage in the region of 3.5V or so. In order to maximize the light output from the LED, and minimize flicker in the resulting light output, it is normal to drive the LED with a constant direct current (DC). It is also possible to use multiple colors of LEDs (typically red, green, and blue LEDs) to generate a broad spectrum of light without the use of a phosphor, although this is not common.
The light emitted by an LED is much more strongly correlated with the current flowing through the LED than with the voltage dropped across the LED. In order to more closely match the light output from multiple LEDs, it is more effective to match the current provided to each LED than to match the voltage provided to each LED. Therefore, it is preferred to connect several LEDs into a series configuration (an LED “string” as shown in item 110 in FIG. 1) than to connect several LEDs in parallel (as shown in item 120 in FIG. 1). An additional benefit of connecting LEDs in series is that the driving voltage is higher (N×˜3.5V, where N is the number of LEDs in series), and the total driving currents are lower, thereby allowing for a more efficient design for the driving circuit.
The power supply for an LED light bulb is typically an alternating current (AC) voltage from a power mains supply. In various parts of the world this supply will be a sine wave with a voltage of 120V RMS (which is equal to 170V peak, or 340V peak-to-peak) or 240V RMS (which is equal to 340V peak, or 680V peak-to-peak), and a frequency of approximately 50 Hz or 60 Hz. The LED light bulb takes energy from this AC supply, and converts it to a form that is suitable for the LEDs to consume.
Additionally, the LED light bulb will need to have a power factor (PF) of close to 1.0, which means that the current taken from the mains supply should be in direct proportion to the voltage of that supply. Expressed another way, this means that the LED light bulb should present a purely resistive load to the mains supply.
This requirement significantly complicates the design of an LED light bulb. If an LED light bulb has a PF of 1.0, then the energy taken from the mains supply will mostly be concentrated into short periods aligned with the peaks of the voltage from the mains supply (item 210 in FIG. 2), but the energy consumed by the LEDs will ideally be at a constant level (item 220 in FIG. 2). Achieving this requires energy storage for short periods of time inside the LED light bulb.
One other important metric of the performance of light bulbs is flicker, which is the variation in the light output from the light bulb over short periods of time. The human visual system is not especially sensitive to flicker above about 20 Hz, as evidenced by the fact that video displays often display video content with a 24 Hz frame rate, at which point most people cannot visually detect that the display is flickering, although displays often operate at multiples of this frequency in order to make flickering even less apparent. At lower frequencies however, say 10-15 Hz, flicker becomes very detectable (and annoying) for the human visual system, and has been known to cause health effects in some cases.
Incandescent light bulbs do not demonstrate significant flicker. The energy supplied to the light bulb by an AC supply occurs primarily concentrated in short bursts corresponding to the peaks of the AC voltage, however the filament retains heat for a long enough period that it smoothens out these peaks to generate light at an almost constant level.
CFL light bulbs also do well with flicker management, because the phosphor demonstrates something called phosphor persistence, where the monochromatic photons of light absorbed by the phosphor are emitted as broad spectrum light after a random delay, leading to a smoothing effect on the level of light output and reducing flicker. Since LED lights use a similar approach with a phosphor converting a monochromatic photon source into broad spectrum light, their phosphor has a similar flicker smoothing effect.
A challenge with LED light bulbs today is heat dissipation. Incandescent light bulbs emit a large percentage of the energy they consume as waste heat, however most of the waste heat is emitted as infra-red light, and they do not contain any especially heat sensitive components, so they do not require specialized heat handling features. Per unit of useful light emitted, LED light bulbs waste less energy as heat; however most of this heat is not radiated and needs to be conducted away from the LEDs using a heat sink. Because of this, LED light bulbs are typically dominated by their heat sinks, and this contributes a significant proportion of the manufacturing cost of a current LED light bulb.
The performance of LEDs is getting better, and the amount of light that they output per unit of energy consumed is going up. Current lab designs of LEDs emit as much as 2× more lumens of light per watt than the LEDs typically used in current production LED light bulbs. When these LEDs are used in production products, their 2× higher light output will mean that they generate less waste heat per LED, but more importantly, their higher light output will mean that only half as many LEDs are required for the same level of light output. Thus, a 2× improvement in light output per watt consumed will result in an approximately 3× decrease in waste heat per LED light bulb.
Improvements in this area will have a large impact in the heat dissipation required in LED light bulbs. As the efficiency of the LEDs themselves continues to improve, this heat dissipation problem will become much less severe, and the need for extensive heat handling will become much less significant.
The other remaining problem with current LED light bulbs is that they require power conversion electronics to convert the power supplied by the mains connection to a form that can be used by the LEDs themselves. The LED electronics also are responsible for managing other functions, such as managing the power conversion electronics to keep the power factor as high as possible, adjusting the light output of the LEDs based on the operation of a dimmer switch, adjusting the light output of the LEDs to account for aging and temperature effects, and other functions. The LED electronics add cost and size to LED light bulbs. They also contribute to waste heat when the power conversion electronics don't convert energy with perfect efficiency.